The present invention concerns detectors for measuring the bactericidal output from low pressure mercury vapor lamps.
Short wave Ultraviolet (UV) radiation having a wavelength in the range of 200-295 nanometers with a peak at about 270 nanometers is capable of destroying bacteria. Any fluid, such as air or water, when exposed to such radiation, is readily sterilized and disinfected without the need for heat or chemicals.
The low pressure variety of mercury vapor lamps, commonly known as germicidal lamps, produce a predominent spectral emission line at 253.7 nanometers and have therefore gained wide acceptance as an economical, efficient source of bacteriastat radiation. Though widely used in water sterilizers and the like, they suffer from several serious disadvantages; the intensity of the 253.7 nanometers emission varies with temperature, voltage, frequency of excitation and bulb life-time. It has therefore been found desirable to provide a means to accurately and continuously monitor the intensity of the 253.7 nanometer emission line and thus the bacteriological effectiveness of the low pressure mercury vapor lamps over long periods of time.
Several methods have been proposed to measure UV radiation over a narrow bandwidth, though most are subject to degradation with age by the incident UV radiation.
Generally, UV detectors consist of a filter or combination of filters to pass only the UV radiation of interest followed by means to convert such radiation into electrical signals. Although sensors do exist which are capable of converting short wave UV radiation directly into electrical signals, they are usually considered too expensive and cumbersome to employ. A more commonly used method employs modern solid state sensors with a phosphor converter screen which, when irradiated with short wave UV radiation, re-emits light with a longer wave length that lies within the detection range of the solid state sensor.
An example of this type of device is described in U.S. Pat. No. 2,349,754 issued to Thomas R. Porter on May 23, 1944. The Porter device utilizes a Corning black glass UV filter preceding the phosphor to filter out the unwanted radiation.
Another device described in U.S. Pat. No. 3,838,282 issued to Phillip B. Harris on Sept. 24, 1974, utilizes an interference filter prior to the phosphor.
Sensors of the type described above employing filters prior to the phosphor converter layer are subject to significant drift with age due to solarization of the filters and windows employed. Compensation for these errors or periodic re-calibration is thus generally required to maintain a reasonable degree of accuracy over long periods of time.
Another method commonly employed is to perform the measurements intermittently. U.S. Pat. No. 3,629,587 issued to Jean A. Decupper on Dec. 21, 1971, describes a shutter system positioned in front of the sensor and opened only when a measurement is to be made, thus shielding the sensor from prolonged UV exposure, and lengthening its useful life.
The methods proposed to date all seek to filter all unwanted frequencies prior to the detection means and thus are subject to the problems associated with such pre-filters.
Since U.V. radiation is harmful to living tissue, most systems employing germicidal lamps such as water sterilizers, use such lamps in a closed optical environment i.e. the emitted light is contained within the system. Thus, conversely, light is not allowed to enter the system, leaving the germicidal lamp as the sole source of radiation.
Low pressure mercury vapor lamps designed for germicidal applications have a well known emission spectrum. In addition to the predominant 253.7 nanometer spectral emission, emission lines extending into the visible region and some infrared frequencies are emitted.